All you need to do is to ensure that your log subscriber is added to
Rails::Subscriber, as in the second line of the code above. The test
helpers are responsible for setting up the queue, subscriptions and turning
colors in logs off.

The messages are available in the @logger instance, which is a logger with
limited powers (it actually does not send anything to your output), and you
can collect them doing @logger.logged(level), where level is the level used
in logging, like info, debug, warn and so on.